Cathodic electrophoretic lacquering (CEL) is a process which, above all, is frequently applied for the purpose of priming automobile bodies, in which process synthetic resins having cationic groups and capable of being diluted with water are deposited with the aid of direct current on bodies that conduct electricity. This principle is described in the literature and is widespread in practice. With this process, a workpiece with a surface that conducts electricity, made of metal or an electrically-conductive synthetic material, is placed into an aqueous electrophoretic bath, is connected as cathode to a source of direct current, and then lacquer is coagulated on the surface by allowing a current to flow (EP-B-0 066 859, EP-A-0 004 090, DE-A-32 15 891). The electrophoretic bath consists of an aqueous dispersion, e.g. a suspension or emulsion, or of an aqueous solution of one or several binding agents which have for example been made water-dispersible by the partial formation of salt with organic or inorganic neutralising agents and, optionally, of pigments, fillers, additives, solvents and other agents dispersed therein.
Lacquer materials of this composition should result in smooth surfaces being formed after deposition and crosslinking of the resulting film. These surfaces conceal imperfections from the lower layers evenly. They frequently serve to produce a ground for subsequent coatings which are intended to produce a high-quality surface lacquer. In practice, however, surface imperfections frequently occur in the stoved CEL film, in particular craters or uneven surfaces. The causes of these coating imperfections may lie in the electrophoretic lacquering materials used, but it is often the case that these imperfections stem from impurities introduced into the electrophoretic-lacquering bath. Examples of impurities from the lacquering material are gel particles arising from production of the binding agent, damaged resin particles arising from production of the pigment paste, impurities in the pigments, as well as foreign bodies from equipment serving to produce suitable materials. Foreign bodies introduced subsequently are, e.g. deep-drawing greases, corrosion-protection greases, seam sealants and substances from previous treatment.
A further class of impurities are those which after deposition of the electrophoretic lacquer coating are deposited from the air on the lacquer film which as yet is not crosslinked, for example aerosols containing fluorine or silicone, as well as lubricants from the transport systems which are necessary for movement of the parts to be coated.
These materials can get onto the lacquer film prior to stoving and then during stoving can cause, for instance as a result of incompatibility, surface imperfections such as craters. Impairment caused by these materials cannot be foreseen but has to be determined by experiment.
These surface imperfections require costly further treatment in order to achieve a smooth surface of the subsequent layers. In order therefore to ensure continuous production it is necessary to avoid surface imperfections of this type. Since it is rather difficult to prevent the many causes of craters, attempts are generally made to avoid crater formation by the use of additives.
Known additives for avoiding the formation of craters are, e.g. silicone oils or organic substances containing fluorine. Although these suppress crater formation in the applied coating, they frequently result in considerable surface imperfections in subsequent layers. Besides, it can be ascertained that the subsequent layers adhere poorly to such undersurfaces. For this reason these additives are not suitable for CEL coatings.
Hence attempts have been made to find other additives which do not contain silicone, in order to avoid these frequent surface imperfections. In U.S. Pat. No. 4,810,535 reaction products of polyoxyalkylene polyamine with epoxy resins are described as additives to CEL systems. It is intended thereby to avoid cratering. In EP-A-0 324 951 polyoxyalkylene polyamines are described as an additive to CEL baths in order to obtain film surfaces free of imperfections. In DE-A-38 30 626 modified acrylates are described as additives to CEL baths, which can be added to the lacquer material in a form which may or may not be neutralised. Also in this case it is intended to avoid surface imperfections of various types. In EP-A-0 301 293 cathodic electrophoretic baths are described which contain homopolymers or copolymers of an alkylvinyl ether by way of additive. In EP-A-0 422 533 cationic (meth)acrylic copolymers as anti-cratering agents for CEL baths are described. It is a feature of all these materials that they have to be used in relatively large quantities in order to have an anti-cratering effect. The problem in this connection is that adhesion to subsequent layers, e.g. fillers or polyvinylchloride materials, is clearly impaired.
Independently of the cause of the craters it can be established that the degree of efficiency of the known anti-cratering agents for CEL systems is inadequate in practice. Even increased quantities of these additives do not result in total elimination of cratering. Therefore the substrates frequently have to be subjected to further treatment, e.g. by grinding or by application of further coatings.
In the as yet unpublished DE patent application 40 18 876 there are described as anti-cratering agents mixtures of polyvinyl ethers with a weight average molecular weight of 500-10000 together with polyesters having a weight average molecular weight of 1000-10000 and an acid number of up to 3, which contain .beta.-hydroxyalkyl ester groups and are preferably not wholly compatible with the CEL binding agent.
Since craters are brought about by the most diverse causes and various causes of cratering often occur at the same time in practice, or various additional substances which cause cratering are added in the course of the lacquering process, there is a need for further anti-cratering agents which can be used in small quantities and which can eliminate the most varied causes of craters.